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Birth month affects risk of developing dementia later in life

By Jo Marchant

The month of your birth influences your risk of developing dementia. Although the effect is small compared to risk factors such as obesity, it may show how the first few months of life can affect cognitive health for decades to come.

Demographers Gabriele Doblhammer and Thomas Fritze from the University of Rostock, Germany, studied data from the Allgemeine Ortskrankenkasse – Germany’s largest public health insurer – for nearly 150,000 people aged 65 and over. After adjusting for age, they found that those born in the three months from December to February had a 7 per cent lower risk of developing dementia than those born in June to August, with the risk for other months falling in between.

There’s nothing astrological about the effect, however. Instead, birth month is a marker for environmental conditions such as weather and nutrition, says Gerard van den Berg, an economist at the University of Bristol, UK, who studies the effects of economic circumstances on health.

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Summer-born babies are younger when they face the respiratory infections of their first winter, for example. And in the past, babies born in spring and summer would have been in late gestation when the supply of fresh fruit and vegetables from the autumn harvest would have largely run out. Pollution from wood fires or coal heating might also have played a role.

There’s evidence from other studies that such factors can have lifelong effects on metabolism and the immune system, increasing the risk of conditions such as diabetes, obesity and high blood pressure. Doblhammer and Fritze’s results show this is true for dementia too.

Dementia on the rise

Other early-life shocks such as recession and famine are known to damage later cognitive health, but these tend to have long-lasting effects on people’s circumstances and lifestyles. The link to birth month is important, says Doblhammer, because it pinpoints the significance of the first few months of life.

An estimated 37 million people worldwide suffer from dementia, and that number is expected to double every 20 years, say the researchers. But although you can’t change your birth month, as far your individual dementia risk is concerned, “it also matters what you do during the rest of your life”, Doblhammer says.

The researchers say the study can’t tell us anything directly about the mechanisms underlying the correlation between birth month and later dementia risk – but they point to several possibilities. For example, poor nutrition might impact directly on brain development at a critical time.

It’s also known that infections brought on by poor nutrition or experienced very early in life – for instance, in a baby’s first full winter – might cause epigenetic changes that affect metabolism and inflammation levels throughout life. This would increase the risk of chronic conditions such as obesity and high blood pressure, which are known to increase the risk of dementia.

Lifestyle changes aimed at lowering dementia risk are often aimed at people in mid or later life. But Doblhammer says tackling the rising incidence of dementia may require early-life interventions too – for example, programmes to improve the nutritional health of young mothers.

Tom Russ, a psychiatrist at the Alzheimer Scotland Dementia Research Centre in Edinburgh, UK, agrees. “It is never too early to start thinking about reducing the risk of developing dementia,” he says.